Printed electronics may be viewed as an extension of the well-known printed circuit board technology. A printed circuit board includes a substrate with conductive metal paths for wiring. All other electronic components are made separately and are soldered or clamped to the substrate of the printed circuit board. In more recent developments, semiconductor integrated circuits are produced to perform most electronic circuit functions. Production of integrated circuits, however, requires the use of specialized semiconductor substrates and is expensive and impractical for producing ultra low-cost electronic devices. For example, typically, the cost of integrated circuits cannot be lowered significantly and even a relatively low level of cost cannot be achieved unless the integrated circuits are produced in a very high volume because of the very high cost of an integrated circuit fabrication line.
Printed electronics are formed by printing images layer by layer, i.e., by depositing one or more layers of material on a wide variety of substrates including uncoated or coated paper, laminated paper products, various plastic films such as polyethylene or polynaphthalene, etc. With printed electronics technology, it is possible to produce microelectric components of an electronic circuit. Some examples of microelectric components that may be produced include transistors, capacitors, resistors, diodes, and light emitting diodes, while examples of complete circuits include RFID tags, sensors, flexible displays, etc. As an example, a capacitor can be constructed by depositing a conducting area, followed by depositing a larger insulating layer and then another conducting area. This process can be repeated to obtain higher capacitance. As another example, an organic field-effect transistor (OFET) can be formed by depositing a conductor layer forming source and drain electrodes, a semiconductor layer, an insulating (dielectric) layer, and then another conductor layer forming a gate electrode.
Especially when low-cost conducting and semi-conducting materials, such as organic polymers, are used as the materials to be deposited, printed electronics forming complete functional circuits (e.g., RFID tags) may be produced at a very low cost on the order of about one-tenth of the cost of producing analogous integrated circuits. Such low-cost printed electronics are not expected to compete directly with silicon-based integrated circuits. Rather, printed electronics circuits may be produced to offer lower performance (e.g., lower frequency, lower power, or shorter lifetime) at much lower cost as compared to silicon-based integrated circuits.
Printed electronics components are made using a set of materials, typically five to seven different materials. These are usually liquids with dissolved and/or suspended polymers, polymer precursors, inorganic materials, and organic or inorganic additives, and are deposited in a wet printing process. Typical wet printing methods include letterpress printing, screen printing, and ink jet printing. Specifically, these materials are deposited in a desired sequence on a substrate and are often cured or activated by a thermal cycling and/or humidity treatment using a convection oven or by use of visible or invisible light, usually protecting the curing polymer in an inert atmosphere. This curing process drives off the solvents in the liquid polymer mix.
As described above, OFETs are devices that are compatible with manufacture by printing. OFET devices are known in the prior art, and one type of OFET, a top-gate OFET, will now be described. FIGS. 1 and 2 illustrate a top view (FIG. 1) and a side view (FIG. 2) of “traditional” organic field-effect transistors (OFET). Referring now to the prior art OFET shown in FIGS. 1 and 2, the OFET 100 is built on a foundation of a substrate 105 comprised of any material capable of supporting the following layers. Representative substrate materials include polymers, semiconductor and insulator wafers, and crystals. Upon the substrate 105, a source electrode 115 and a drain electrode 120 are patterned using techniques known to those of skill in the art (e.g., photolithography). The source 115 and drain 120 electrodes are typically metals, but can also be conducting organic materials. A semiconductor layer 110 having substantially planar upper and lower surfaces and uniform thickness, typically composed of a polymer or small molecule semiconductor, is deposited between the source electrode 115 and drain electrode 120. An insulating layer 125 of uniform thickness is deposited upon the source electrode 115, drain electrode 120, and semiconductor layer 110. Finally, a gate electrode 130 is deposited on the insulating layer 125 such that the gate electrode 130 is substantially aligned with the area of the semiconductor layer 110 (e.g., the gate electrode 130 spans the semiconductor layer 110 between the source electrode 115 and the drain electrode 120).
Charge carrier mobility is indicative of how well a charge can pass through the semiconductor in an OFET, and thus is one figure of merit that characterizes OFETs. Higher mobility is indicative of improved device performance. It has been reported that the channel semiconductor charge carrier mobility is partially dependent on the self-organization of the macromolecules in the semiconductor polymer as it makes the transition from liquid polymer layer to solid polymer. In addition to the material properties of the polymer itself, the factors that affect this self-organization are the temperature and cooling rate of the polymer during the curing process and the nature of the atmosphere around the polymer during curing. As is currently known, one method to produce a channel with high mobility utilizes a slow cooling rate to produce a solidified semiconductor polymer channel with higher structural order and, therefore, greater charge carrier mobility.